The thin film transistor liquid crystal display (TFT-LCD) is a well-developed product now. However, because a thin film transistor (TFT) device is produced on a glass-insulated panel, the related ESD problems occur during the manufacturing process of the panel. Once there is a transistor or more devices of the active matrix in the panel damaged by the ESD, it will result in a point defect or a line defect on the display area of the panel. Therefore, the panel is classified imperfect product and degrades the yield rate seriously.
During the mass-production processes of TFTs (array, cell, and module process), the ESD occurs very easily. The electrostatic discharge causes the insulated layer of the matrix-array TFT's gate electrode to be broken and very large leakage current. Accordingly, the occurrence of the electrostatic discharge affects the yield rate of a product very much. The design of electrostatic discharge protection can protect the devices of active matrix from the ESD due to external causes such as human or equipments so as to promote the effect of protection for transistor devices and achieve better ESD protection.
As shown in FIG. 1, the well-known design for electrostatic discharge protection uses two transistors in a diode style by reversed connection to form the ESD protection unit 10. Each signal line 12 (scan line or data line) connects to the protection unit 10 such that the signal line 12 is protected. In the circuit, one end of the transistor T11 connects to the IC pad, the signal line 12, and the the transistor's T21 source electrode; the other end of the transistor T11 connects to the common electrode 13 and the drain's transistor T21 source electrode. The discharge paths for electrostatic discharge are that the transistor T11 is the device for discharging positive electrostatic discharge, and the transistor T21 is the device for discharging negative electrostatic discharge. Although the protection unit 10 can achieve the effect of electrostatic discharge protection, the effectiveness is not satisfactory because only one ESD protection unit 10 provides discharge paths for each signal line 12.
Regarding the manufacturing process of TFTs, the cell test function process is a very important step of testing panels. It can help to find fail panels; the cell test function design of testing panel can avoid the unnecessary cost for module assembly in the later stage. For the time being, there are two major methods for the cell test function. The first one is the shorting bar method: connecting all scan lines to data lines respectively by shorting bars, and then conducting a voltage to perform panel testing. However, once the cell test process is completed, the shorting bars connected between scan lines and data lines have to be cut by a laser cut machine so as to let the scan lines be independent to each other. The data lines are independent to each other, too.
The second one is the TFT switch method. Please refer to FIG. 2, outside the matrix area 20 that is used to set the display pixel, a TFT switch element 22 is set between each scan line and data line, and then connects to the display matrix area 20. Connecting the gate electrode of each TFT switch element 22 is going to input a voltage signal to the test-switch pad PDSA. Besides, the drain electrode of each TFT switch element 22 connects to the corresponding test pads PSE, PSO, PDR, PDG, and PDB, which are distinguished by odd, even, R, G, and B. The source electrode of each TFT switch element 22 connects to the scan line and the data line, and then inputs to the display matrix area 20. As long as a display voltage is applied to test pads PSE, PSO, PDR, PDG, PDB, and the common electrode (Vcom), together with a voltage is applied to the test-switch pad PDSA so as to let the TFT switch element 22 be “ON”, the cell test function can be achieved. Although this method does not like the first method that needs a laser machine to cut, the electrostatic discharge protection circuit has to be extra manufactured to achieve the effect of ESD protection.